There are primarily 2 elements involved in every MapReduce operation: The Mapper and the Reducer.
The Mapper assigns every key to a value and creates a key-value pair. The Reducer takes the key-value pair and aggregates it (reduce it into single key-value pair tuple).
The Combiner acts as a mini-reducer. The primary job of a combiner is to process data from the mapper before assigning it to the reducer. The use is purely optional and depends on how much time efficiency you require in certain operations. You should learn more about how to implement a combiner via a good hadoop certification course.